1. Field of the Invention
This invention relates to a magnetoresistance element suitably used for a magnetic encoder, and more particularly, to a magnetoresistance element which can stably output a signal which is varied in a sinusoidal manner with the rotation of a rotor magnet, necessary for analyzing an output signal of a magnetic encoder in high resolution using an analog method, irrespective of its attached position or the pattern of the magnetic poles provided in the rotor magnet.
2. Description of the Prior Art
Recently, there has been widely employed a magnetic encoder instead of an optical encoder employed conventionally. Such a magnetic encoder mainly comprises a rotor 1 having a multipolar magnet formed on the outer circumference thereof, and a magnetoresistance element 2 disposed at the stator side, as shown in FIG. 1. In the figure, reference numeral 3 represents a shaft fixedly connected to the rotor 1; 4, a bearing for the shaft 3; 5, a case of the magnetic encoder; and 6, a base plate on which is provided a signal processing circuit for processing the output signal of the magnetoresistance element 2, and so on.
Usually, the magnetoresistance element 2 includes four magnetoresistors a.sub.1, a.sub.2, b.sub.1 and b.sub.2 on a same substrate so that respective phase differences between the magnetoresistors a.sub.1 and a.sub.2, and between the magnetoresistors b.sub.1 and b.sub.2 become 180 degrees (.pi. radians) in electrical angle as shown in FIG. 2, or the phase difference beween the magnetoresistors a.sub.1 and b.sub.1 becomes 90 degrees (.pi./2 radians). The respective one ends of the magnetoresistors a.sub.1 and a.sub.2 are connected to a common electrode C.sub.a, the respective one ends of the mangetoresistors b.sub.1 and b.sub.2 are connected to a common electrode C.sub.b, and the respective another ends of the magnetoresistors a.sub.1, a.sub.2, b.sub.1 and b.sub.2 are connected to respective electrodes L.sub.a1, L.sub.a2, L.sub.b1 and L.sub.b2, respectively. In FIG. 2, an imaginary line represents an object to be detected such as the rotor 1. The rotor 1 has many magnetic poles so formed on the outer circumference thereof that N-pole and S-pole are arranged alternately, and moves in the direction indicated by an arrow P.
FIG. 3 shows a circuit for obtaining two output signals A and B whose phases are different from each other, by using the magnetoresistance element 2 having the configuration shown in FIG. 2. In the figure, OP.sub.1 and OP.sub.2 represent comparators; R.sub.1 to R.sub.4, resistors; and T.sub.A and T.sub.B, output terminals of the output signals A and B, respectively.
Conventionally, the resolving power of the magnetic encoder has been improved by reducing the pitch of the magnetized pattern in the multipolar magnet of the rotor 1 as well as the respective distances between the magnetoresistors a.sub.1 and a.sub.2, and between the magnetoresistors b.sub.1 and b.sub.2 of the magnetoresistance element 2. However, if the pitch of the mangetized pattern is reduced, the range of the magnetic field produced by the magnetic poles decreases. For this reason, the magnetoresistance element 2 must be disposed close to the rotor 1. In this case, high accuracy is required in the components used for the encoder and in assembling of the encoder. Therefore, the encoder is more expensive to manufacture and the risk of damaging the encoder increases due to contact between the rotor 1 and the magnetoresistance element 2 from to vibration or force exerted to the rotational axis of the encoder.
In a positional sensor referred to as a synchronous resolver, a method has long been employed for improving the resolving power using natures of trigonometric functions.
FIG. 4 shows an example of a circuit for the magnetic encoder to which the above method is applied. In the figure, reference numeral 11 reprents an oscillator; 12, a counter; 13, a sin-ROM (Read Only Memory) in which sine values have been stored corresponding to the count values of the counter 12; 14, a cos-ROM in which cosine values have been stored corresponding to the count values of the counter 12; 15 and 16, digital-to-analog (D/A) converters; 17 to 20, amplifiers; 21 to 24, variable resistors; a.sub.1, a.sub.2, b.sub.1 and b.sub.2, magnetoresistors; 25 and 26, resistors; 27, a comparator for detecting a zero-cross point; and 28, a D-flip flop.
In operation, the oscillator 11 generates pulses in turn at a predetermined timing and the counter 12 counts the number (for example 0 to 255) of the pulses supplied from the oscillator 11. The count value of the counter 12 is supplied to the sin-ROM 13, the cos-ROM 14 and the D-flip flop 28, respectively. When the count value of the counter 12 is supplied to the addresses of the sin-ROM 13 and the cos-ROM 14, corresponding sine and cosine digital values are read out from the data of the sin-ROM 13 and the cos-ROM 14, respectively, and are supplied to the digital-to-analog converters 15 and 16, respectively. The digital-to-analog converters 15 and 16 convert the input digital values to analog values. The output of the digital-to-analog converter 15 is supplied to the amplifiers 17 and 18, respectively, and the output of the digital-to-analog converter 16 is supplied to the amplifiers 19 and 20, respectively.
The amplifiers 17 and 19 amplify the input analog values and then supply the amplified signals to the electrodes L.sub.a1 and L.sub.b1 via the variable resistors 21 and 23, respectively. Accordingly, AC voltage signals in proportion to sin.omega.t and cos.omega.t (where .omega. is angular frequency) are applied to the electrodes L.sub.a1 and L.sub.b1, respectively. On the other hand, the amplifiers 18 and 20 invert the phases of the input analog values and amplify the phase-inverted analog values, and then supply them to electrodes L.sub.a2 and L.sub.b2 via the variable resistors 22 and 24, respectively. Accordingly, AC voltage signals in proportion to (-sin.omega.t) and (-cos.omega.t) are applied to the electrodes L.sub.a2 and L.sub.b2, respectively. The voltage at the point C.sub.a which corresponds to the common electrode of the magnetoresistors a.sub.1 and a.sub.2 is added to the voltage at the point C.sub.b which corresponds to the common electrode of the magnetoresistors b.sub.1 and b.sub.2, and the thus added signal is supplied to the positive input terminal of the comparator 27 via the resistor 25. The negative input terminal of the comparator 27 is connected to the earth. The comparator 27 detects a zero-cross point on the basis of the above signal obtained by adding the voltages at the point C.sub.a and C.sub.b, and the D-flip flop acts to take in signals on the basis of the detected result of the comparator 27.
Assuming that if DC voltage signal is applied to the magnetroresistors a.sub.1 and a.sub.2, and b.sub.1 and b.sub.2, the voltage at the point C.sub.a is proportional to cos.theta. and the voltage at the point C.sub.b is proportional to sin.theta., where .theta. is a rotational angle of the rotor 1 represented in electrical angle in radians provided one pitch of the magnetic poles of the rotor 1 is 2.pi.. Under the condition, if AC voltage signal is applied to the magnetoresistors a.sub.1 and a.sub.2, and b.sub.1 and b.sub.2, new AC voltage signal whose phase is shifted by .theta. relative to that of the initally applied AC voltage singal can be obtained by adding respective voltages at the points C.sub.a and C.sub.b, which is expressed as follows: ##EQU1## where K is a constant.
Accordingly, even small rotational angle corresponding to less than one pitch of the magnetic poles can be detected by detecting the phase difference between the applied signal and the added signal.
The above equation (1) can be applied accurately to the case only where the resistance of the magnetoresistance element 2 varies sinusoidally with the rotational angle of the rotor 1. However, as a result of observing the voltages at the points C.sub.a and C.sub.b when applying DC voltage across the magnetoresistance element 2, it has been found that the form of the obtained output voltage curve is considerably distorted from a sinusoidal waveform when the magnetoresistance element 2 is set close to the rotor 1 on the one hand, and the form of same is similar to the sinusoidal waveform but the output voltage decreases and is liable to be affected by noise when the magnetoresistance element 2 is set apart from the rotor 1.